Large-scale MIMO systems or massive MIMO systems were firstly introduced in [1] where each BS is equipped with dozens to several hundreds of transmit antennas. One main advantage of such systems is the potential capability to offer linear capacity growth without increasing power or bandwidth [1][4], which is realized by employing Multi-User MIMO (MU-MIMO) to achieve the significant higher spatial multiplexing gains than conventional systems. In this system, the BS groups UEs at each scheduling slot and transmits data to them on the same time and frequency resource.
It has been proved that Zero-Forcing (ZF) precoding with a total transmitting power constraint is almost the best choice to maximize the sum rate for large-scale MU-MIMO systems [2]. However, in practice, the power of each antenna is restricted instead of the total power. It means that maximizing the power utilization requires the sum power of all users at each antenna to be the same. Unfortunately, it is generally not the case in practice, because of the randomness of the ZF precoding matrix. As a result, it causes a dilemma to the BS: on the one hand, ZF precoding could not fully use the transmit power, which leads to throughput loss; on the other hand, full power utilization means that there exists residual interference among the grouped users, since the ZF precoding matrix is violated, which also results in throughput loss. Conjugate Beamforming (CB) is another practical precoding method for MU-MIMO precoding in large-scale MIMO communication systems because of its simplicity for implementation. Similarly to ZF, CB also faces the optimal power allocation problem when the power of each antenna is restricted. Therefore, more sophisticated power allocation methods are needed to maximum the sum rate of MU-MIMO systems. Due to the aforementioned reasons, this invention provides four different methods to allocate the power to each data stream based on two different optimization objectives when ZF precoding is employed by the BS. In addition, a simple power allocation method is also offered when CB is employed by the BS. The advantages of this invention include: 1. when ZF precoding is employed, two of the four power allocation methods have better performance than the rest two in the low Signal-to-Noise Ratio (SNR) region and vice versa, so different power allocation methods could be employed in different SNR regions to achieve the maximum sum rate of MU-MIMO systems; 2. when CB is employed, a very simple power allocation method could be employed with little sum rate loss; 3. the sum rate losses of all methods provided in this invention are negligible compared to the case where the total transmitting power instead of the per-antenna power is constrained; 4. most importantly, these methods are not affected by a scaling factor of each channel vector so channel estimation with an arbitrary scaling factor would be sufficient, which alleviates the accuracy requirement of channel measurement in massive MIMO systems.